1. Field of the Invention
The present invention relates to a differential STBC (Space-Time Block Code) transmitting/receiving apparatus that can be applied to a multiple antenna wireless communication system using eight or less transmit antennas.
2. Description of the Related Art
Compared to the wired channel environment, the radio channel environment has low reliability due to multipath fading. Yet, in the radio channel environment, especially using multiple transmit antennas, the use of STBCs in a transmitter achieves a transmit diversity gain, thereby overcoming the fading and considerably increasing the reliability of the transmitted signals. These advantages cause the STBCs to be widely used.
If radio channel information is known to a receiver, conventional STBCs are used. In a real system, the radio channel information is estimated using training symbols.
However, a transmission scheme may be preferable which does not utilize the radio channel information in the case where it is not easy to estimate the radio channel information in the receiver or the estimation of the radio channel information is accompanied by too much overhead due to fast varying radio channels.
To use such a transmission scheme, a differential STBC transmitting/receiving apparatus was proposed. V. Tarokh and H. Jafarkhani proposed a differential STBC transmitting/receiving apparatus for a single transmit antenna in “A Differential Detection Scheme for Transmit Diversity”, (IEEE J. Select. Areas Commun., Vol. 18, pp. 1169-1174, July 2000). They extended the idea of differential modulation used in a communication environment using a single transmit antenna to a conventional coherent differential STBC transmitting/receiving apparatus. V. Tarokh and H. Jafarkhani extended the scheme further to systems with more transmit antennas (i.e. 2, 3 and 4 transmit antennas) in their later thesis “Multiple Transmit Antenna Differential Detection from Generalized Orthogonal Designs”, (IEEE Trans. On Info. Theory, Vol. 47, pp. 2626-2631, September, 2001).
The differential STBC transmitting/receiving apparatus shows a performance degradation of about 3 dB and a similar decoding complexity, compared to the existing STBC transmitting/receiving apparatus.
However, Jaehak Chung, et. al. proved in “Performance Evaluation of Multiple Transmit Antenna Non-Coherent Detection Schemes in Fast Fading Channel”, (Wireless Personal Communications, Vol. 26, Issue 2-3, pp. 249-265, 2003) that in real implementation, a complex signal cannot be transmitted with the 8×4 STBC design proposed in “Multiple Transmit Antenna Differential Detection from Generalized Orthogonal Designs”.
It is thus concluded that the Tarokh's scheme is viable for two transmit antennas in the case of transmitting a complex signal and for 2, 3 or 4 transmit antennas in the case of transmitting a real signal.
FIGS. 1A and 1B are block diagrams of a conventional differential STBC transmitting/receiving apparatus in a multiple antenna transmitting/receiving system.
FIG. 1A is a block diagram of a transmitter having N transmit antennas 106-1 to 106-N. Referring to FIG. 1A, the transmitter is composed of a symbol mapper 101 for generating k symbols, P1, P2 . . . Pk from the input binary data b1b2 . . . bi by mapping every 2b bits to one symbol, a vector generator 102 for generating vector components for the input of a signal received from an adder 104, multipliers 103-1 to 103-k for multiplying the symbols received from the symbol mapper 101 by the vectors, the adder 104 for adding the outputs of the multipliers 103-1 to 103-k, and a space-time block encoder 105 for generating a STBC using an encoding matrix whose entries are the sum received from the adder 104 and providing the STBC to the respective transmit antennas 106-1 to 106-N.
The symbols P1, P2 . . . . Pk carry real information. For better understanding of the Tarokh's scheme disclosed in “Multiple Transmit Antenna Differential Detection from Generalized Orthogonal Designs”, it will be described in the context of k=4. Using three transmit antennas amounts to deleting a signal to be transmitted through a fourth transmit antenna.
When a complex signal is transmitted in a differential STBC through four transmit antennas, one differential STBC block is equal in length to 8 symbol periods. Thus, the space-time block encoder 105 uses such a transmission matrix as illustrated below in Equation 1. The transmission matrix is characterized by orthogonality between columns.
                    [                                                            x                1                                                                    x                2                                                                    x                3                                                                    x                4                                                                                        -                                  x                  2                                                                                    x                1                                                                    -                                  x                  4                                                                                    x                3                                                                                        -                                  x                  3                                                                                    x                4                                                                    x                1                                                                    -                                  x                  2                                                                                                        -                                  x                  4                                                                                    -                                  x                  3                                                                                    x                2                                                                    x                1                                                                                        x                1                *                                                                    x                2                *                                                                    x                3                *                                                                    x                4                *                                                                                        -                                  x                  2                  *                                                                                    x                1                *                                                                    -                                  x                  4                  *                                                                                    x                3                *                                                                                        -                                  x                  3                  *                                                                                    x                4                *                                                                    x                1                *                                                                    -                                  x                  2                  *                                                                                                        -                                  x                  4                  *                                                                                    -                                  x                  3                  *                                                                                    x                2                *                                                                    x                1                *                                                    ]                            (        1        )            
In a first block, arbitrary symbols S1=(s11, s21, s31, s41)T are mapped to the positions of x1, x2, x3 and x4 in the above matrix. This operation is carried out in the Space-time block encoder 105. The symbols of the first block are generated irrespective of the input binary data. Hence, the first block symbols carry no information. The signals of subsequent blocks can be recursively expressed as follows.
For a (v+1)th block, four symbols SV+1 are obtained by multiplying symbols P1 to Pk carrying information by vectors V′k(Sv) and then summing the products. The vectors V′k(Sv) have the four symbols Sv=(s1v, s2v, s3v, s4v)T transmitted in a vth block. The four symbols Sv+1 are expressed as shown in Equation 2:
                              S                      v            +            1                          =                              ∑                          k              =              1                        4                    ⁢                                    P              k                        ⁢                                          V                k                ′                            ⁡                              (                                  S                  v                                )                                                                        (        2        )            and V′k(Sv) is given as in Equation 3:V′1(Sv)=(s1v,s2v,s3v,s4v)V′2(Sv)=(s2v,−s1v,s4v,−s3v)V′3(Sv)=(s3v,−s4v,−s1v,s2v)V′4(Sv)=(s4v,s3v,−s2v,s1v)  (3)
The thus-obtained Sv+1=(s1v+1, s2v+1, s3v+1, s4v+1)T are mapped to x1, x2, x3 and x4 in the above matrix.
FIG. 1B is a block diagram of a receiver with M receive antennas 107-1 to 107-M. The receiver is comprised of a vector generator 108 for receiving a STBC from the transmitter through the receive antennas 107-1 to 107-M and generating vectors, a permuter & delay 109 for permuting and delaying the STBC, a plurality of multipliers 110-1 to 110-k for multiplying the outputs of the permuter & delay 109 by the vectors received from the vector generator 108, a plurality of filters 111-1 to 111-k for estimating symbols by band-pass-filtering the outputs of the multipliers 110-1 to 110-k, and a symbol demapper 112 for demapping the estimated symbols to binary data.
In operation, the permuter & delay 109 permutes eight received signals in a vth block and the vector generator 108 generates vectors composed of eight received signals in a (v+1)th block. The outputs of the permuter & delay 109 are multiplied by the vectors, thereby producing P1 to P4. The received signals can be expressed as in Equation 4:r1v=h1s1v+h2s2v+h3s3v+h4s4v+w1v r2v=−h1s2v+h2s1v−h3vs4v+h4s3v+w2v r3v=−h1s3v+h2s4v+h3s1v−h4s2v+w3v r4v=−h1s4v−h2s3v+h3s2v+h4s1v+w4v r5v=h1s1v*+h2s2v*+h3s3v*+h4s4v*+w5v r6v=−h1s2v*+h2s1v*−h3s4v*+h4s3v*+w6v r7v=−h1s3v*+h2s4v*+h3s2v*−h4s2v*+w7v r8v=−h1s4v*−h2s3v*+h3s2v*+h4s1v*+w8v  (4)where hi denotes the channel gain between an ith transmit antenna and the receiver. By permuting r1v, r2v, r3v, r4v, r5v, r6v, r7v and r8v, the permuter & delay 109 outputs Rv1 to Rv4 as shown in Equation 5:
                                                                        R                v                1                            =                              (                                                      r                    1                    v                                    ,                                      r                    2                    v                                    ,                                      r                    3                    v                                    ,                                      r                    4                    v                                    ,                                      r                    5                                          v                      *                                                        ,                                      r                    6                                          v                      *                                                        ,                                      r                    7                                          v                      *                                                        ,                                      r                    8                                          v                      *                                                                      )                                                                                                                                    ⁢                              =                                                                            (                                                                        s                          1                          v                                                ,                                                  s                          2                          v                                                ,                                                  s                          3                          v                                                ,                                                  s                          4                          v                                                                    )                                        ⁢                                          H                      ⊥                                                        +                                      (                                                                  w                        1                        v                                            ,                                              w                        2                        v                                            ,                                              w                        3                        v                                            ,                                              w                        4                        v                                            ,                                              w                        5                                                  v                          *                                                                    ,                                              w                        6                                                  v                          *                                                                    ,                                              w                        7                                                  v                          *                                                                    ,                                              w                        8                                                  v                          *                                                                                      )                                                                                                                                                                      ⁢                                                =                                                                                                              V                          1                                                ⁡                                                  (                                                      S                            v                                                    )                                                                    ⁢                                              H                        ⊥                                                              +                                          W                      1                                                                      ,                                                                                                        R                v                2                            =                              (                                                      -                                          r                      2                      v                                                        ,                                      r                    1                    v                                    ,                                      r                    4                    v                                    ,                                      -                                          r                      3                      v                                                        ,                                      -                                          r                      6                                              v                        *                                                                              ,                                      r                    5                                          v                      *                                                        ,                                      r                    8                                          v                      *                                                        ,                                      -                                          r                      7                                              v                        *                                                                                            )                                                                                                                                    ⁢                              =                                                                            (                                                                        s                          2                          v                                                ,                                                  -                                                      s                            1                            v                                                                          ,                                                  s                          4                          v                                                ,                                                  -                                                      s                            3                            v                                                                                              )                                        ⁢                                          H                      ⊥                                                        +                                                                                                                                                    ⁢                              (                                                      -                                          w                      2                      v                                                        ,                                      w                    1                    v                                    ,                                      w                    4                    v                                    ,                                      -                                          w                      3                      v                                                        ,                                      -                                          w                      6                                              v                        *                                                                              ,                                      w                    5                                          v                      *                                                        ,                                      w                    8                                          v                      *                                                        ,                                      -                                          w                      7                                              v                        *                                                                                            )                                                                                                                                    ⁢                                                =                                                                                                              V                          2                                                ⁡                                                  (                                                      S                            v                                                    )                                                                    ⁢                                              H                        ⊥                                                              +                                          W                      2                                                                      ,                                                                                                        R                v                3                            =                              (                                                      -                                          r                      3                      v                                                        ,                                      -                                          r                      4                      v                                                        ,                                      r                    1                    v                                    ,                                      r                    2                    v                                    ,                                      -                                          r                      7                                              v                        *                                                                              ,                                      -                                          r                      8                                              v                        *                                                                              ,                                      r                    5                                          v                      *                                                        ,                                      r                    6                                          v                      *                                                                      )                                                                                                                                    ⁢                              =                                                                            (                                                                        s                          3                          v                                                ,                                                  -                                                      s                            4                            v                                                                          ,                                                  -                                                      s                            1                            v                                                                          ,                                                  s                          2                          v                                                                    )                                        ⁢                                          H                      ⊥                                                        +                                                                                                                                                    ⁢                              (                                                      -                                          w                      3                      v                                                        ,                                      -                                          w                      4                      v                                                        ,                                      w                    1                    v                                    ,                                      w                    2                    v                                    ,                                      -                                          w                      7                                              v                        *                                                                              ,                                      -                                          w                      8                                              v                        *                                                                              ,                                      w                    5                                          v                      *                                                        ,                                      w                    6                                          v                      *                                                                      )                                                                                                                                    ⁢                                                =                                                                                                              V                          3                                                ⁡                                                  (                                                      S                            v                                                    )                                                                    ⁢                                              H                        ⊥                                                              +                                          W                      3                                                                      ,                                                                                                        R                v                4                            =                              (                                                      -                                          r                      4                      v                                                        ,                                      r                    3                    v                                    ,                                      -                                          r                      2                      v                                                        ,                                      r                    1                    v                                    ,                                      -                                          r                      8                                              v                        *                                                                              ,                                      r                    7                                          v                      *                                                        ,                                      -                                          r                      6                                              v                        *                                                                              ,                                      r                    5                                          v                      *                                                                      )                                                                                                                                    ⁢                              =                                                                            (                                                                        s                          4                          v                                                ,                                                  s                          3                          v                                                ,                                                  -                                                      s                            2                            v                                                                          ,                                                  -                                                      s                            1                            v                                                                                              )                                        ⁢                                          H                      ⊥                                                        +                                                                                                                                                    ⁢                              (                                                      -                                          w                      4                      v                                                        ,                                      w                    3                    v                                    ,                                      -                                          w                      2                      v                                                        ,                                      w                    1                    v                                    ,                                      -                                          w                      8                                              v                        *                                                                              ,                                      w                    7                                          v                      *                                                        ,                                      -                                          w                      6                                              v                        *                                                                              ,                                      w                    5                                          v                      *                                                                      )                                                                                                                                    ⁢                              =                                                                                                    V                        4                                            ⁡                                              (                                                  S                          v                                                )                                                              ⁢                                          H                      ⊥                                                        +                                                            W                      4                                        .                                                                                                          (        5        )            [CAN THIS BE PROVIDED AS TEXT AND NOT AN IMAGE?]where H⊥ is defined as shown in Equation 6:
                              H          ⊥                =                  [                                                                      h                  1                                                                              h                  2                                                                              h                  3                                                                              h                  4                                                                              h                  1                  *                                                                              h                  2                  *                                                                              h                  3                  *                                                                              h                  4                  *                                                                                                      h                  2                                                                              -                                      h                    1                                                                                                -                                      h                    4                                                                                                h                  3                                                                              h                  2                  *                                                                              -                                      h                    1                    *                                                                                                -                                      h                    4                    *                                                                                                h                  3                  *                                                                                                      h                  3                                                                              h                  4                                                                              -                                      h                    1                                                                                                -                                      h                    2                                                                                                h                  3                  *                                                                              h                  4                  *                                                                              -                                      h                    1                    *                                                                                                -                                      h                    2                    *                                                                                                                        h                  4                                                                              -                                      h                    3                                                                                                h                  2                                                                              -                                      h                    1                                                                                                h                  4                  *                                                                              -                                      h                    3                    *                                                                                                h                  2                  *                                                                              -                                      h                    1                    *                                                                                ]                                    (        6        )            
The permutation values are multiplied by the signals received at the (v+1)th time, that is, the vectors from the vector generator 108 and the real parts of the products are calculated by Equation 7:
                                                                        Re                ⁢                                  {                                                            R                                              v                        +                        1                                            1                                        ⁢                                          R                      v                                              k                        *                                                                              }                                            =                              Re                ⁢                                  {                                                                                    S                                                  v                          +                          1                                                T                                            ⁢                                              H                        ⊥                                            ⁢                                              H                        ⊥                        *                                            ⁢                                                                                                    V                            k                            ′                                                    ⁡                                                      (                                                          S                              v                                                        )                                                                                                    *                          T                                                                                      +                                          W                      k                                                        }                                                                                                        =                                                2                  ⁢                                                                          ⁢                                                            ∑                                              k                        =                        1                                            4                                        ⁢                                                                                                                                                  h                            k                                                                                                    2                                            ⁢                                                                                          ⁢                                              P                        lk                                                                                            +                                  Re                  ⁢                                      {                                          W                      k                                        }                                                                                                          (        7        )            
The reason for calculating the real parts is that V′k(Sv) is not orthogonal with respect to a complex signal. That is why the conventional differential STBC transmitting/receiving apparatus does not support a complex signal.
If P1 to P4 are elements in a set equally distant from the origin, they can be estimated through threshold detection even though the channel values are not known. This method advantageously enables decoding even though the channel values are not known.
However, to make P1 to P4 have real values and, at the same time, be equally distant from the origin, P1 to P4 must be selected from {1, −1}. This is equivalent to using BPSK (Binary Phase Shift Keying). When a higher-order modulation scheme is used to increase data rate, joint design of the P1 to P4 is required, thus increasing the complexity of the receiver considerably.